FIG. 1 shows an exemplary appearance of an ultrasonic transducer and an ultrasonic diagnostic apparatus. As shown in FIG. 1 (a), the ultrasonic transducer is connected to the ultrasonic diagnostic apparatus by a cable. The ultrasonic transducer transmits ultrasonic waves in the direction of an arrow shown in the figure, and receives reflection waves which are reflected from a living body and is in the opposite direction of the arrow. As shown in FIG. 1 (b), the ultrasonic diagnostic apparatus performs image analysis on the reflected waves received by the ultrasonic transducer, and displays on a monitor an image of the inside of the living body obtained through the analysis.
When such an ultrasonic transducer transmits the ultrasonic waves from a piezoelectric transducer, the ultrasonic waves are emitted not only to the front side of a transducer, but also to the back side of the transducer.
Here, an example of a conventional ultrasonic transducer is described with reference to a drawing. FIG. 2 is a vertical cross-sectional view for showing a structure of the conventional ultrasonic transducer. FIG. 2 shows the structure in which, from the top, an acoustic lens 3, a matching layer 2, a piezoelectric transducer 1, and a backing layer 4 are stacked.
In typical ultrasonic diagnostic apparatus, the ultrasonic waves transmitted from the piezoelectric transducer 1 passes through the matching layer 2 and the acoustic lens 3, and then emitted into a living body. As a result, the ultrasonic waves reflected within the living body pass through the same route in the reverse order as the outgoing ultrasonic waves passed, and then received back by the piezoelectric transducer 1. Depending on the strength of reception or response time, a received signal is visualized in shading by the ultrasonic diagnostic apparatus.
On the other hand, as described in the beginning, ultrasonic waves having an opposite phase to the phase of ultrasonic waves that are emitted to the front are emitted from the piezoelectric transducer 1 to the back at the same time. The ultrasonic waves emitted to the back side of the piezoelectric transducer 1 are attenuated by the backing layer 4. However, when the backing layer 4 is formed of the material which does not sufficiently attenuate the ultrasonic waves emitted to the back side, the ultrasonic waves having the opposite phase can reflect within the backing layer 4 and can go back toward the piezoelectric transducer 1.
Due to the effect of the reflected waves described above, noise is superimposed on the received ultrasonic wave signal, resulting in a deteriorated ultrasonic wave resolution of the ultrasonic diagnostic apparatus.
In general, the ultrasonic transducer includes in the backing layer 4 a material having internal loss and distance that can provide adequate attenuation to the ultrasonic waves emitted to the back side, and thus enables the ultrasonic diagnostic apparatus to maintain satisfactory ultrasonic wave resolution. However, such a conventional method has a disadvantage of increasing the thickness of the backing layer itself.
Furthermore in Patent Literature (PTL) 1, in order to decrease the thickness of the backing layer 4, a heat release block is disposed on the back side of the backing layer 4, thereby attenuating the ultrasonic waves emitted to the back side of the piezoelectric transducer 1.